Field buses or process networks are used for real time data transmission between actuators and sensors, especially in the field of automation technology. The data have to be transmitted via the field bus in ensured fashion within an established time span, so that field buses make a high demand on the latency for data transmission. The Ethernet is also increasingly being used as a field bus, a broad use of data transmission protocols from the TCP/IP protocol family being striven for. The Ethernet standard specifies various data transmission speeds, such as Ethernet networks having data transmission speeds of 10 Mbit per second, 100 Mbit per second and 1 Gbit per second. Moreover, there are Ethernet networks in which the data are transmitted in a wireless manner via radio. What is common to these Ethernet networks is the decoupling of the physical data transmission and the protocol layer, which states how the respective data packets are structured. The physical data transmission is implemented by semiconductor IC's and an appropriate wiring of the units or modules connected to one another. The protocol layer is identical for all types of transmission. In the case of the Ethernet, the protocol layer, in turn, includes an abundance of various data transmission protocols, such as TCP/IP, FTP, HTTP. Usually, a microprocessor combines the data packets corresponding to the desired data protocol, and sends them to the physical data transmission unit. In the field of real time transmission, particularly in measured data recording in the industrial field, for controlling machines or systems, in which the predictability of the data transmission is important, the Ethernet has not yet been widely used as the transporting medium. Because of the use of switches and the TCP/IP or UDP protocol with regard to a real time capability, the Ethernet is little deterministic and has a greater jitter than usual field bus systems.
FIG. 1 shows a conventional Ethernet topology. In this instance, several measuring units MG are connected to a data processing unit, such as a PC, via an Ethernet switch. The Ethernet switch represents a multiplexer which analyzes the data on the Ethernet and routes them on appropriately. If several measuring units simultaneously send data in the direction towards the PC, collisions take place which the Ethernet switch is only partially able to balance out. It cannot be ensured, in this context, that measured data which are transmitted by measuring unit MG to the Ethernet switch arrive at the PC even in response to the first data transmission attempt. When a collision does occur, the data are sent again by measuring unit MG. In the course of that, there comes about an unknown delay time between the creation of the data in the measuring unit MG and the arrival of the data at the PC. This unknown delay time depends on how many measuring units MG are connected to the Ethernet switch and what data volume the individual measuring units MG have at the various points in time. Since renewed sending of the data by measuring unit MG is required under certain circumstances, the measured data further have to be temporarily stored. The star-shaped topology shown in FIG. 1 also has the disadvantage that the wiring expenditure is relatively large.
In the usual Ethernet data transmission systems for transmitting real time data, collisions are avoided by the PC requesting the real time data or the measured data from the individual measuring units MG in a so-called handshake method. For this purpose, an appropriate data memory and an appropriate data management are provided in the various measuring units MG. However, the disadvantage of a data transmission after a handshake method is that the bandwidth provided by the Ethernet is not able to be fully utilized, since either data are sent from the PC to a measuring unit MG or conversely data from a measuring unit MG to the PC. Consequently, the data are transmitted in a half duplex operation. The result is that the data transmission times or transport times in such usual Ethernet field bus systems are not minimal, and are strongly dependent on the number of interconnected measuring units MG. Real time requirements, such as particularly the predictability of the data transmission times and short latency times, are therefore not satisfied in such usual Ethernet systems.